text:
What is the mathematical relationship between mass and speed for constant thrust in level fight?
Imagine that we have a wing with constant thrust in level flight.  Then imagine that we add mass to that wing.  In order to maintain level flight, I would expect that the angle of attack would need to be increased, and that speed would decrease.
As the mass increases, I would also anticipate that speed would decrease and at some point the wing would stall.
What is the mathematical relationship between mass (x), speed (y) and stall speed where all other variables are constants?
Please assume that the thrust vector is always horizontal, and does not change with the angle of attack.

@sophit Thanks, but no, that doesn't help.  I need to be able to set constants and graph mass vs speed and when a stall would occur.

The answer shows a formula that provides a relationship between speed and lift. In steady flight, you know that L = W, which is equal to M*g, so that helps you set up a relationship between speed. You can then plot mass versus speed.
If you manage to do it, you can provide the solution here as an answer to your own question to share the knowledge :)

@sophit The trouble is that I don't know how to calculate the relationship between AoA and lift... or is there no way of calculating it and it's worked out empirically?

For that, you need to know about the airfoil type.
If you know the airfoil, as a first guess you can look up the relationship between AoA and lift for the airfoil. Or you can assume that it has an airfoil of similar aircraft, see The Incomplete Guide to Airfoil Usage

@ROIMaison For my purposes, a flat line aerofoil would do, as I'm really only after an approximation that I can use to make a simplified formula.  I saw https://aviation.stackexchange.com/questions/64490/is-there-a-simple-relationship-between-angle-of-attack-and-lift-coefficient that suggests Cl=Sin(2a)...

You can't answer this question with a simple equation.  For one thing, it would most certainly depend on which side of the power-required curve or thrust-required curve the aircraft was initially flying on.

@MontyWild fortunately, the relationship between AoA and Cl is generally linear within the nonstalled flight regime.  The rest of Cl is dependent on the airfoil. More here.  Just pick your airfoil.

I am going to stay out of this controversy, but the answer you desire is the following...
$(C_L/C_D)\cdot{T} = (C_L\rho{SV^2})/2 = W$

You can't do this.  (see comments below) Expanding on quietflyer's comment, the result (consequences), of adding mass in a constant thrust scenario depends on where the aircraft (wing) is on the power curve. This is a graph representing the relationsip between airspeed (horizontal scale), and the power or thrust required (vertical), to maintain that airspeed. The curve is U-shaped. The two points where the red and blue lines intersect are the only two stable points on the graph.

For all aircraft there is a speed (the bottom or lowest point on the blue U-shaped curve), where the power(thrust) required to maintain that speed is a minimum. I.e., to maintain any faster, (or slower) speed, requires more thrust or power. This is because the drag due to lift increases as AOA increases, and at slow speeds, (high AOA), this increase is greater than the reduction in drag from just going slower.
So, if above, (faster), than that airspeed corresponding to L/Dmax, reducing thrust, (moving the red line down), and slowing down brings you closer to that minimum thrust required airspeed, (assuming you continue to trim to maintain level flight), and then, unless you reduced thrust below the thrust required for that slower speed, when you have slowed to the speed that your reduced thrust can maintain, the aircraft will stabilize.
If, otoh, you start this exercise from below your L/Dmax airspeed, then the scenario you describe would happen.

Yes, you can do this.  Nevertheless, quiet flyer makes a valid statement.  OP's question presents an important quandary:  How do we assess flight performance under specific conditions?  What are the constraints in the mathematics of flight? What are the requirements on paper to answer OP's question?

@ThomasPerry in a practical sense we vary thrust with weight (by way of Velocity) to maintain optimal AoA.  What the OP wants (constant thrust) is not often practiced in aviation any more than it is in trains and automobiles.

I admit that the first sentence in my answer (You can't do this.), might be a bit over the top. Perhaps it should be amended to read "You can't  make these assumptions...", or "you can't have a mathematical relationship ... for constant thrust in level flight" - but "You can't do this." was shorter, and a bit more attention grabbing.

@CharlesBretana your graph more than makes up for it.  It boils down to how much speed can one get for that amount of thrust.  How much weight can one lift at that speed.  The OP hinted at variable AoA.

@sophit and Charles Bretana -- My comment was posted before sophit's answer-- I suspect he is probably right, but I haven't pored through either his answer or this answer carefully enough to be certain either way --

What is the mathematical relationship between mass and speed for constant thrust in level fight?

What you're looking for can be easily obtained from the standard equations for lift and drag.

Thrust $T$ wins drag $D$, which can be as usual expressed as:
$$D=½\rho V²SC_d=T$$
where $\rho$ is air's density, $S$ wing surface and $C_d=C_{d_0}+kC²_l$; $k$ is a constant which depends on the actual wing geometry while $C_{d_0}$ represent the parasite drag of the whole airplane; $C_l$ can be simply derived from the usual equation for lift $L$ that equals weight $W$:
$$L=½\rho V²SC_l=W \Rightarrow C_l=\frac{W}{½ \rho V²S}$$
Substituting we get:
$$T=qS(C_{d_0}+k(\frac{W}{qS})²)$$
which is the equation you're looking for ($q$ is short for $½\rho V²$).

Since you've made the hypothesis of constant $T$, this equation can be rewritten as:
$$q²S²C_{d_0} - qST + kW²=0$$
which is the equation of a quartic in $q$ (i.e. a biquadratic in $V$) whose solution is well known. This solution gives you the requested

relationship between mass and speed for constant thrust

You have an interesting question but the wrong ending.

the maximum weight carrying capacity will be at V min power, not at stall

There is a practical application to the constant thrust/less weight application as the aircraft consumes its fuel.
Now the plane can reduce its angle of attack (and fly faster) because more thrust can be used for forward speed and less for lifting.

Thrust required for lifting = induced drag

Thrust required for forward speed = form drag

As one goes faster, the proportion of thrust for induced drag decreases, and the proportion of thrust for form drag increases.
The sum of these 2 drags forms a minimum at Vbg.
Propeller aircraft have V min power at a V less than Vbg because of greater prop thrust output per RPM at lower airspeeds.  Jets (and gliders) have their maximum weight carrying capacity at Vbg.
Relevant reading here.  Adding weight shifts stall speed, Vmin power, Vbg, and Vy to higher airspeeds.


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Is this blended wing body trijet airliner real?

Is it a commercial plane? Is this a better design in any way than regular engines on the wings?

This looks fake.

It's an Adobe Photoshop Trijet. Was manufactured in Soviet Russia in total secret.

If it would be real, with that main gear so forward, I would expect it to tailstrike on every landing.

ahh so much for whats app and the nice people on the int webs.

Thunderbirds are Go!

With those flaps it is totally unstable in pitch. The tiny elevons could not compensate, even with a super-responsive FCS.

It is a blended body design. It is a hoax.  See similar photo from Popular Science magazine 2003 on wikipedia page: https://en.wikipedia.org/wiki/Blended_wing_body

By Source (WP:NFCC#4), Fair use, https://en.wikipedia.org/w/index.php?curid=37713174

it's not so much a hoax as a concept under study at the time that led nowhere.

The photograph depicting it as a commercial airliner is a hoax. The aircraft design, is not a hoax. But the question was regarding a "trijet airliner."

Boeing released some concept art showing what the concept would look like as an airliners. I'm not sure if the photograph you show is a hoax or one of those images.

Though the aircraft is not real, it is apparently based on the Boeing X-48B Blended wing body experimental vehicle.

According to NASA,

Potential benefits of the aircraft include increased volume for carrying capacity, efficient aerodynamics for reduced fuel burn and possibly significant reductions in noise due to propulsion integration options..

Also, a lot of middle seats!

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sort: votes


text:
What is the size of toilet or septic tanks in various planes?
This question, led me to wonder, how big are the tanks that hold poo. I had seen a BBC video, showing how they empty the tank on an A380 at Heathrow, can't remember the capacity.
Also, what about planes like Cessna etc, do they have toilets?
In an emergency can the tank be emptied, in case it overweight, i.e. above the Maximum Landing Weight?

Related: What would non-potable water be used for on an airliner?

The A380 has a total waste tank capacity of 2096L or 554 US gal, according to Airbus.

B. Capacity
  There are four waste tanks, two upper deck tanks and two main deck tanks, see FIGURE
  5-4-10-991-003-A.
  (1) Upper Deck Waste-Tanks
  - 373 l (99 US gal).
  Each tank is precharged with 35 l (9 US gal) of chemical fluid.
  (2) Main Deck Waste-Tanks
  - 675 l (178 US gal).
  Each tank is precharged with 35 l (9 US gal) of chemical fluid.
  (3) Total Waste Tank Capacity
  - 2096 l (554 US gal).

The size will vary for each aircraft type, the Boeing 747 had a capacity of only 1000L.
Smaller General Aviation aircraft do not have lavatories, this depends on aircraft size and purpose. The average Cessna will not have lavatories either, pilots have to plan their stops properly or rely on accessory such as the TravelJohn. Larger General Aviation aircraft such as smaller jets and aircraft designed for more than 4 passengers are more likely to have lavatories on board.
(Image Source: Siebert Aero)
There is no dumping system for waste tanks, mostly for hygienic and environmental reasons, as the waste tanks are not only filled with fecal and thus biological matter, but also with chemicals.

Also, there is no possibility of the aircraft becoming overweight during flight due to waste tanks filling up. Whatever goes into those tanks was on the plane at take off, whether in the galley, a carry-on, or a person. If it wasn't overweight at take-off, transferring the location of the weight won't make it become so during flight.

@FreeMan An aircraft can be below MTOM/MTOW, but once airborne it can be above MLW. It cannot technically have more weight than before someone used the lavatory, there you are right...

Yes, a trip to the loo is a simple weight redistribution.

Airlines also should provide travel john's along with the puke pouch, for cases of emergency..

In modern airliners,the toilet seat sits directly on the holding tank    It is covered by a shroud. Imagine a tank sitting on the floor of the bathroom  and the shroud around it. That is the size of the tank. About the size of a large fishing cooler.
The tank holds a mixture including formaldehyde. A contained pump with a screen continually recycles the fluid over and over..  The inlet has a screen in it to keep most of the solids out, the pump macerates any leftovers.
The tanks are evacuated and filled through plumbing that leads to ports on the aircraft exterior skin. The unit is modular and when empty surprisingly light.
Couple of side notes. When a plane is down and sitting in the sun the smell permeates the whole cabin. The shrouds around the toilet seat becomes extremely nasty yet was a favorite place for smuggling cocaine in the 80's.

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Are aircraft windows UV-ray proof?
As we know, at higher altitudes the dose of UV rays increases, and planes usually spend a larger time there, then why aren't windows of airplanes made UV proof. A simple coating on the inside might solve the problem.
Relevant question

are you asking if they are, or do you already know the answer and are asking why is it so?

I would like to know are they UV proof.. Older aircrafts may not be, but maybe some new aircraft might have some coating.. And if they are not, then why don't they address the issue.

@Firee The top answer to the other question explains it quite clearly: Yes, they are enough UV-proof. I fail to see what you want to achieve.

@yo: Read the edit on the answer you are referring to. It says it needs some kind of coating. So, do aircraft manufacturers apply any kind of coating?

Ah ok. Still I don't think it makes sense to ask this question; it would be more sensible to update the answer to the other question.

Then please consider revising the wording, in your title you ask if, but when you say then why aren't you are assuming they aren't.

Are you asking about cockpit windows or cabin windows?

Can't find a specific reference right now but afaik aircraft that fly at higher altitudes have windshields made of laminate safety glass. The same kind as in a car windshield just much thicker. Laminate glass blocks nearly all uv. I don't know about the cockpit side windows, tho. The side windows in a car are usually tempered glass which does not block uv unless it's coated or tinted. Tempered glass is stronger and harder to break but shatters into dangerous shards and upon breaking offers no resistance to penetration. That's why car windshields are required to be laminate

@Firee, the accepted answer of the question you yourself link clearly explains that the two most common materials for aircraft windows provide UV protection as inherent property of the materials. The last sentence of not being UV-hard refers to the material itself needing specific treatment if it is to be used in application where it will be exposed to sunlight.


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What do pilots do, when there is a long wait before take-off, after gates are closed?
Pilots are supposed to carry out the mandatory checks before the flight take-off or taxi. But what happens when there is a delay, when the aircraft cannot push back from the gate, due to non-clearance from ATC, or due to heavy traffic etc.

Do the pilots, re-run the checks, or carry out more extensive checks of the aircraft?
Do the pilots just chit-chat, eat, or make phone calls to their family?
Is there any standard procedure to be followed during this wait time?

Edit: As someone tagged this as duplicate, my question no where suggests time spent by pilots during long haul flight, its about what they do when the pushback is delayed. Hope it helps.

They are constantly on the phone with company, handling, airport, ATC, etc. to try to get going. It is extremely frustrating for pilots to get delayed, so they do everything they can to avoid it.

Possible duplicate of (in a general sense) How do long-haul pilots cope with boredom?

@Dave it's related but I think the types of things the pilots have to do on the ground would be different from during cruise.

@fooot Agreed I have removed the vote but ill leave the comment as i think it directs to some relevant material.

@fooot exactly, I provided an edit in my question. Also I don't understand, when only 1 person suggest this is duplicate, how come the question suddenly carries the warning, isn't it supposed to happen after 5 votes?

Most seem to have tablet flight bags these days, so they could watch an airline disaster movie.

A possible duplicate comment appears at any time a duplicate is suggested. It will not be closed as a duplicate until five closevotes happen.

Apart from what Daniel listed already, they're also nervously watching the clock.
If they're late by too much they may miss their mandatory crew rest period and have to cancel the flight or hand over to another crew.
This can of course get very expensive for the airline, especially if all or most passengers have to be accommodated in hotels overnight for a departure the next day in case no replacement crew can be arranged or the forced delay causes the flight to be unable to take off due to closing of the departure airport before departure or closing of the destination airport before arrival there.

I've been on many flights with delays, and here's what I've seen/heard. They check often with ground/tower for estimated wheels up time, and any restrictions on slots available at their destination airport. They also check fuel so that if it drops below reserves, they can call to return to the gate for more fuel. Other than  that, just idle chatter I guess.


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Is the Boeing 777 low speed warning same as the master caution warning?
Is the Boeing 777 low speed warning the same as the master caution warning?
If the answer is no, I would appreciate an answer with a link to a recording of the low speed warning so I can tell the difference.

The caution message AIRSPEED LOW comes with the normal master caution beeper:

Airspeed Low
The EICAS caution message AIRSPEED LOW is displayed and the box around
the current airspeed indication on the PFD is highlighted amber if airspeed is
below minimum maneuvering speed. [...]

Message
Level
Aural
Message Logic

AIRSPEED LOW
Caution
Beeper
Airspeed is below minimum maneuvering speed.

(Boeing 777 FCOMv2 15.20.9 - Warning Systems - System Description)
This has been criticized because the beeper might not be enough to draw the flight crew's attention to the low speed in an emergency situation:

Boeing also has a "caution tone" for low airspeed on the 777, along with versions of the 747, according to an FAA panel studying the issue. But other planes have a verbal warning such as "airspeed low." Clifford said the 777 tone "has proven to not capture the pilot's attention in these low airspeed circumstances." [...]
Airbus has a warning that says "speed speed speed" every five seconds between 2,000 and 100 feet when an aircraft goes too slow.

(USA TODAY)

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What are the advantages and disadvantages of a double bubble fuselage?
Double bubble is mostly used for Cargo Aircraft with 2 intersecting circles which is in other words: they are 2 floor which are used for more space for freight and also passengers. this type of fuselage mostly used in the past by some small airlines but today they are only used for cargo. i wonder to know all the things above mentioned by me are right or not?
and the other question is that what ARE advantages and disadvantages of A380 fuselage? and how it is designed that able to carry a large number of passengers although its fuselage is not double bubble?

Welcome to Aviation. SE! I cleaned up your question a little bit. Use the edit button if you want to make additional changes or additions.

I feel your "other question" should be asked separately, but we'll see what others think.

I agree with @Federico, the two questions are distinct and should be asked separately.

I agree as well.

if you think these are 2 separate questions, you should close as too broad, @GdD

@EfeBallı same for you

Double bubble is the lightest way to make a pressurized fuselage taller than it is wide.  It maintains the pressure hull elements, skins and formers, closest to pure tension, like a balloon, so they can be a bit lighter since they see little bending stress from pressure (only the interconnecting floor beams see significant bending loads, from the weight of the payload).
The A380 went with an ovalized cross section because it provides a better packaging compromise when you want two decks of roughly the same floor area, although it would be somewhat heavier than a DB cross section because the frame elements of the partially flattened sides are subject to a certain amount of bending because the pressure loads are trying to circular-ize the oval (the floor beams help but the oval sections away from the beams are still subject to that stress).
Think about what would happen if you really cranked up the pressure inside the A380, and imagine how the pressure hull would deform under stress; it would swell out to a more circular shape except where retained by the floor beams, before going "POP" (you'd end up with a kind of "triple bubble" before it went bang).  That deformation load trying to make the oval a circle has to be absorbed as bending loads in the frames, so they have to be heavier than they would be if they were already purely circular.
The packaging advantages over a pure double bubble make it worth the weight penalty.
The original double bubble pressure hull was the Curtiss C-46, which was originally designed as a pressurized airliner (otherwise they would have used an oval cross section), and used the configuration to make a pressure hull that was a light as possible while keeping it relatively narrow.  Then you had the Boeing 377 Stratocruiser, which also went that way in order to graft a passenger cabin on top of a B-29 fuselage and wing without making it too heavy.

"only the interconnecting floor beams see significant bending loads, from the weight of the payload" - however, the bending is limited by the crosswise tension in the floor, which resists the outward pressure on the walls. The floor's loadbearing capability actually increases with altitude, which can be useful if you hit an air pocket.

I'll add two other reasons for the A380's double bubble design.
The A380's fuselage needs to bear much larger vertical bending loads compared to horizontal, the wing lift (which is sometimes greater than the aircraft weight) and the main landing gear impact are two forces that bend the fuselage upwards from the center. The nose landing gear impact and horizontal stabilizer forces are two smaller forces that create vertical bending. Whereas the horizontal (side to side) bending forces are limited to gust loads on the fuselage (pretty minor), forces from the vertical stabilizer (which is actually bending+torsional) and nose landing gear forces from steering on the takeoff/landing roll and taxiing. So it pays to have a larger MOI for vertical bending. If we compare a  circular, vertical (top and bottom) double bubble and side to side double bubble of the same cross section, the vertical double bubble will have the highest MOI for vertical bending.
Another concern is operational, it is easier to connect multiple jetways to one side of the aircraft on the upper and lower decks for faster loading. Higher class passengers also like being separated from the plebs by using their separate jetways to reach their part of the aircraft, which is often the upper deck for the A380. Twin aisles on both decks will also help with passengers reaching their seats easier. Other fuselage cross sections would require multiple aisles (of which only the outer ones would help with evacuation) or large amounts of scooting and "sorry, may I?" moments in a seat row.
Speaking of evacuation, having lots of exits and placing people as evenly as possible between exits is easiest with a vertical double bubble design. With other cross sections, people closer to the center will take longer to reach an exit.


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Can a commercial airplane overtake another airplane going in the same direction?
If there are two commercial airplane flying towards the same destination, on the same flight path, and the second plane is being faster than the first plane, is the second plane allowed to overtake the first airplane?
If so, how close are two planes allowed to fly while overtaking each other?

The sky is 3D..

In many cases it will literally overtake

Faster aircraft often overtake slower on the same routing. This is usually accomplished by flying at higher or lower altitudes. This is called vertical separation. They can also be separated laterally by radar vectors so that they are 3-5 nautical miles apart. The exact distance will depend on the ATC standards for the particular airspace.
Often as they approach the destination ATC will ask one aircraft to speed up and one to slow down slightly. This can sometimes allow the faster aircraft to pass the slower aircraft, but it also insures they have the required separation when they are on approach to land.

One of the benefits of vertical separation is that aircraft can be much closer to each other than the miles required for lateral separation. Besides, larger aircraft tend to stick to routes and corridors. Lateral overtaking would be a bit of a hassle in crowded areas.

First of all, if there are two commercial airplane flying towards same destination, in same flight path there will 2000 feet difference in altitude and can be overtaken if the 2nd aircraft got greater tailwind than 1st aircraft. Hope it’s clarify.

1000 ft if RVSM is in use

@ymb1 Though there are exceptions, such as the North Atlantic Tracks, where it's 1,000' same direction (because nearly all of the traffic is going the same direction at a given time of day.)

@reirab: Isn't that because the North Atlantic Tracks have many miles of lateral separation, if not entire degrees, between the east- and westbound tracks? IIRC this is not for safety reasons, but because the tracks are chosen with regard to prevailing winds.

@MSalters That's often true. But they're also just not active at the same times of day. At any rate, my point was just that RSVM sometimes does mean 1,000' separation for same-direction traffic.

Aircraft obviously fly at different speeds which you can see in the Flight tracking app. Aircraft also sometimes fly along the same paths following navigational beacons. When they are not under air traffic control they need to be able to overtake one another. In this event, 14 CFR 91.113(f) specifies that the faster-moving aircraft must overtake on the right-hand side, or starboard side, of the slower aeroplane.


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Do Instrument Landing Systems restrict building heights and can these be overcome?
A recent territorial spat between the countries Malaysia and Singapore over an ILS implementation made me curious. I'm interested in finding out about ILS and how an implementation of one would affect the surrounding urban restrictions.
In that specific example, one country is saying that the ILS implementation would stifle its economic growth by restricting building height if the flight path went according to the yellow path showed on this picture

The question is, can the ILS still work if the flight path is altered to, say, an angled landing going through the water (along the thin white line) from North to South? Is it possible for a plane to turn and land into the airport (and bypass the land, hence removing building height limits) while still benefiting from the ILS in this case?
Note: The airport is 2km away from the other side of the shore (which is a separate country)

You can delete your own question.

I can't recall the airport, but I think I have seen at least one where the ILS system does not actually guide the aircraft to the runway, but instead to a critical turning point in the runway approach.

This is a fairly interesting case as it crosses international borders. Technically, as a sovereign nation they are free to build and approve construction as they see fit. Depending on where you are in the world this is handled in all kinds of different ways. Blocking the runway of another nations airport may not be great for international relations but there is nothing stopping them from doing it. There have been some fairly infamous real estate holdouts that test the limits of what you describe.
As for the latter part of your question, there have been instrument approaches that don't necessarily point you right at the runway initially. as @mfarver notes in the comments the classic example of this is the old approach into Kai Tak Airport which basically pointed you to the side of a mountain. The mountain face was painted in a checked pattern, when it came into view a right turn was executed and you were lined up with the runway. The approach is no longer in use but has been pretty notorious, you can see it in action here. With the prevalence of GPS approaches coming into use all sorts of things can be done that cant be done with a traditional straight line radio signal ILS. The issue with your described approach is that the airplane lands on the runway in the reverse direction which may not always be possible with the current winds.
The other option if things do get in the way is to have a really steep, slam dunk style final approach as they do in St. Barts. This, however can lead to other issues.


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Are there other practical means of 'slowing' down an aircraft, other than parachutes?
Whenever an airliner crashes, inevitably questions arise as to how can the planes be made safe and ultimately, how can we avoid loss of lives.
Though statistically, modern airliners are safest ever, but due to the nature and outcome of even a single crash, it affects people psychologically worldwide.
Questions like parachutes for planes or even passengers are numerous on this site. For example, I posted this questions after reading this question.
The way I understand, if an aircraft can be slowed down, by means of a parachute, the resultant damgage to human lives would be a lot less. This led me to wonder, what can be other means of slowing down an aircraft, which would crash otherwise.

what can be other means of slowing down an aircraft, which would crash otherwise. If it's going to crash, what makes you think that a parachute will stop the crash?  Are you able to reference any crash that might have been prevented by a parachute?  Remember, the big problem is that if you slow it down more than about 140kts, which is going to be fatal, it will fall out of the sky because you slowed it down.

Voting to close as Too Broad because this kind of question doesn't fit in the Stack Exchange paradigm. SE is designed so that every question should have one answer that is more 'right' or 'acceptable' and therefore chosen as accepted. This question could very easily have two (or more) equally valid and acceptable answers, therefore must be considered as Too Broad. OP, please rewrite to be less so. Perhaps come up with an alternate solution idea yourself and present it as a question.

For an aircraft to fly, it needs forward speed. If you slow down an aircraft too much, it will stall and fall out of the sky. Airplanes are designed to not slow down. It's not "if a Boeing 737 is slowed down to 80 knots, more people will survive". A Boeing 737 has to fly at 130 knots minimum. If you want a machine that can, it's a helicopter.
Most crashes happen suddenly, e.g. approaching a runway while not paying attention to airspeed. It's not like "Hmmm, I think it's going to crash in 3 minutes, let's arm this system to minimize injury". It's like, "we're stable, runway ahead, everything's good......(terrain! terrain! PULL UP!)......oh sh!t ! Max power!!!". If the pilot can foresee the accident, most of the time he can avoid it altogether.
If you know well in advance you ARE going to crash, chances are the aircraft is completely out of control, spinning around and falling. In this case I think a large enough parachute will be effective.

I would think a plane completely out of control has most likely suffered loss of some pretty important control systems, who is to say that the parachute will deploy? And of course, a tumbling aircraft is probably more likely to cut and destroy the parachute.

There are a few things you could do but a full frame chute would be the most effective.
Speed brakes and flaps (really anything that creates drag) can act to slow a plane down even in a glide (engine out) situation.
In the realm of more unlikely things some sort of rocket style short burn booster can be deployed and discharged apposing the direction of movement thats how we landed on the moon. But these would add weight to the aircraft and could (if deployed incorrectly) put the aircraft in a worse situation. This does NOT include deploying thrust reversers that can be dangerous.
Slipping an aircraft (rudder opposite aileron deflection) can aid in slowing an airplane down. All student pilots in the US are required to know how to do this. Its strange the first time but quite interesting to execute.

Please explain how a chute could prevent an airliner crash,

@Simon There's other questions about that.

In the realm of "totally impossible" we also get the Kulbit, which is quite effective for slowing the aircraft down but rather tricky for an airliner to pull off.

@raptortech97 - Isn't that a post-stall maneuver, like Pugachev's Cobra, that requires vectored thrust to accomplish?

@KeithS yes. As I said, rather tricky for an airliner to pull off.


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How the placement of the pitot and static ports decided?
Forgive the this topic placement as it might be similar to other older ones (Although I a bit sure that the main contents might be different than other older ones).
In the upcoming days I will have a very important interview to a position I am interested as it is involved around a passion of mine.
I would like to gain some insights in the following topics (any research paper, final thesis, or other explanatory material is more than welcome).

Pitot and static sensors placement. How do we choose the most ideal position on an aircraft's structure to place these sensors? What do we need to take into consideration apart from document standards?
Definition of air data related measurement points and routing of pressure tubes involved. Calibration of air data systems.

Any help is much appreciated,
Thanks in advance.

Welcome to aviation.stackexchange!

Welcome! As for the placement I bet (hence no answer) it's mostly based on experience. The calibration is obviously done according to test flight data.

Potential duplicate: https://aviation.stackexchange.com/questions/56631/does-the-pressure-at-the-static-ports-drop-as-the-aircrafts-speed-increases/56640#56640

Both the pitot and static pressures will vary a lot based on:

Airspeed
CG location
Angle of bank
Angle of attack
Configuration, especially flaps

In the CFD model, and then again in flight test, you are looking for a placement that generates the least variation in calibration error over the greatest range of conditions.
No matter where you place them, you are going to calibrate the instrument to correct  for the physical limitations of the measurement system.  So you don't care how large the calibration error is, you care much more about how consistent it is.
Designers, by necessity, accept a lot of calibration error in the pitot (airspeed) system and publish various speed in KIAS to accomodate that.
But the static port drives altimetry and really needs to be placed where static pressure remains the most consistent across a wide range of attitudes and configurations.

You put the pitot inlet so its tip protrudes into the free stream air, outside of the boundary layer flow and normal to the direction of travel.
The static port inlet is positioned flush to the fuselage surface so a pressure reading taken from it is exactly equal to the existing ambient pressure at zero velocity. The easiest way to do that is by experiment.
(an aside: a friend who homebuilt a Rutan Q2 claimed he got a 250 MPH cruise on an O-200 engine. I told him, "yeah, and if you let ME locate your static port, I could get you 300!")


text:
How can TAS be calculated from indicated temperature and Mach number?
Situation
I am working on programming my own E6B while I learn more about aviation, but I have run into some confusion. Two pairs of the equations are very similar except for using true or indicated temps based on which equation. If anyone has a Sporty's E6B, it is listed as Planned Mach # and Actual Mach # as well as Planned TAS and Actual TAS.
Important Info
The inputs for the Mach # formulas are as follows:
Temp, Mach #
While the output is:
TAS
The inputs for the TAS formulas are as follows:
Pressure Altitude, Temp, CAS
While the outputs are:
TAS, Mach #, Density Altitude
"Planned" versions take "True Temp" according to the manual, while the "Actual" versions use "Indicated Temp". From what I can tell from research, True temp is known as SAT (Static) and Indicated would be RAT (Ram). I have also been told that there usually isn't a difference between the 2 unless you are over certain speeds... but I am thinking that maybe there is some form of temperature conversion that I need to do with the RAT/Indicated input. As you can see below, using the same inputs will provide slightly different outputs.
Examples
Planned Mach # with Indicated Temp of 22C and Mach # of 0.33 provides a True Airspeed output of 221
Actual Mach # with Indicated Temp of 22C and Mach # of 0.33 provides a True Airspeed output of 218.6
Where I'm at and have tried
I am able to calculate the correct answers when using True Temp but not Indicated. Currently, I've been trying to convert RAT to SAT with some math I found using a "recovery factor". I get quite close to the result that my Digital E6B but it is not the same result. I have used many different formulas from an AI I won't name... and overall I am just stumped. After a week of trying, I am going to ask this forum, which has helped me many times before.
Does anyone have some insight on how to handle this and use the Indicated Temperature in these equations?

Does this answer your question?

Does this, this and this answer your question?

How do you program an E6B?

@MichaelHall I am a Software Engineer, and I am developing an app that amongst other things, contains the equations you would have on a commercially available E6B... that's what I meant by that.

@sophit I ended up using one of the formulas in those which is also about halfway thru the DeltaLima answer below, so yes... kinda. Thank you for the links.

TAS is Mach number ($M_a$) multiplied by the speed of sound ($a$).
$$\textrm{TAS}=M_a\cdot a$$
The speed of sound depends on the temperature of the (static) air $T_s$.
$$ a = \sqrt{\gamma  RT_s}$$
The relation between the measured total air temperature (or ram air temperature) and the static temperature is given by:
$$\frac{T_\mathrm{total}}{T_{s}}={1+\frac{\gamma -1}{2}M_a^2}$$
Since the measurement probe may not recover all the energy from the adiabatic heating, a correction factor is introduced:
$$\frac{T_\mathrm{total}}{T_{s}}={1+\frac{\gamma -1}{2}eM_a^2}$$
This 'recovery factor' $e$ is determined empirically for the temperature sensor used.
Putting it all together yields:
$$\textrm{TAS} = M_a \sqrt{\frac{\gamma R T_{\textrm{total}}}{1+\frac{\gamma -1}{2}eM_a^2}}$$

The ratio of specific heats $\gamma$ is 1.4
R, the specific gas constant is 287.052874 J K/kg

The formula is for SI units, so temperature should be in Kelvin [K], TAS will be given in meters per second [m/s]

I will review it and let you know how it goes... appreciate it

Thanks, @DeltaLima for the in-depth response. Currently, I am unsuccessful with this because after trying the formula you gave by hand, code, and calculator I get a TAS of 30 which I know is not right. I feel like there is nowhere on Earth where one-third of the speed of sound would be 30mph.
I may be misunderstanding the answer you gave but will keep looking into this.

What units did you calculate with? The formulas are for SI units. Sorry, I should have mentioned that.

Oh, yeah definitely was not using those. Thanks for the heads up.

I have gotten to a point where a bit of rounding will basically line up with what I expect for the results. What I ended up doing is using the formula from above that discusses the "relation between" the 2 temps and doesn't use the recovery factor.
I solved for static temp and threw the result into my original equation which I have been using when given True/Static temperature... I will mark this as answered, but I wanted to comment that I did not get here by using the very last equation in your answer.

@Bellarose143 when I put the numbers in, with e = 1 for a perfect temperature sensor, I get almost exactly what you have given in your example. (220.9 for the true temperature case, 218.6 for the indicated temperature case)

I'll try again with 1... I was using 0.98.


text:
How to fill out the Nav Log when reaching set heading point before the top of climb from the departure airport?
My current scenario is that I am flying from airport A to airport B. I need to reach 8000ft from sea level but it takes 22nm horizontally to reach that altitude and my first set heading point is 12nm away from my departure airport A. It is required that my set heading point is 12nm away from the departure airport A because that is where I will be changing track.
Do I split the climb into two or more portions/legs on the nav log along with a top of climb (TOC)?

Welcome to aviation stackexchange. Good first question :)

What requires that your "set heading point" be 12nm from departure?  Is this for a checkride?

No it is not for a checkride, but I need that set heading point because of mountains and other obstructions. Another reason is that I need to change direction when reaching the set heading point.

There is no hard and fast rule, or right/wrong way to complete a Navlog.
For a check ride you would want to be as thorough and complete as possible, doing it just the way you were taught, and probably including a top of climb as a waypoint to track climb vs cruise fuel.
(although the PHAK doesn't mention doing this...)
However, if you aren't creating the log for anybody else but you, just do it in a way that you find most useful.
If you think calculating and displaying a waypoint line in the log corresponding to the top of climb would help in fuel planning or situational awareness, then by all means do it.
The Navlog is just a tool for you to use as you see fit.


text:
What are the differences between types I, II, and III emergency doors?
What are the differences between types I, II, and III emergency doors?

this helps? https://aviation.stackexchange.com/q/12830/1467

The answer suggested by @Federico mentions this FAR: §25.807   Emergency exits for the US/FAA.

Not exactly a duplicate, this question is only for emergency doors, and not limited to the US.

The major difference is size.
FAR part 25.801 defines them as:

(1) Type I. This type is a floor-level exit with a rectangular opening
of not less than 24 inches wide by 48 inches high, with corner radii
not greater than eight inches.
(2) Type II. This type is a rectangular
opening of not less than 20 inches wide by 44 inches high, with corner
radii not greater than seven inches. Type II exits must be floor-level
exits unless located over the wing, in which case they must not have a
step-up inside the airplane of more than 10 inches nor a step-down
outside the airplane of more than 17 inches.
(3) Type III. This type
is a rectangular opening of not less than 20 inches wide by 36 inches
high with corner radii not greater than seven inches, and with a
step-up inside the airplane of not more than 20 inches. If the exit is
located over the wing, the step-down outside the airplane may not
exceed 27 inches.

There is also a type IV:

(4) Type IV. This type is a rectangular opening of not less than 19
inches wide by 26 inches high, with corner radii not greater than 6.3
inches, located over the wing, with a step-up inside the airplane of
not more than 29 inches and a step-down outside the airplane of not
more than 36 inches.

If you read the full text, you will also find definitions for ventral and tailcone exits. For pictures I refer you to this answer.


text:
What does Phase 3A mean for avionics?
I've learned about Phase 1, Phase 2, and Phase 3 electrical systems. But recently I've faced an issue about replacement of Phase 3 Common Display System (CDS) Display Units (DUs) with new Phase 3A CDS DUs.
My question is what does it mean actually regarding Phase 3A? And what are the differences between Phase 3 and Phase 3A?

As there is an upgrade from 3 to 3A possible with a kit and an upgrade, and the FAA notice that orders the phasing out (no pun intended) of the phase 3 displays  was about possible WiFi interference, my guess is that 3A is a emissions-hardened variant of 3

It is just a matter of technology, it turned out that phase 3 are subject to interferences from wireless devises, resulting in disappearance of displayed data.
We have been alerted of this some years ago and as long as several similar equipments are simultaneously  functional, it is probabilistically nearly impossible to loose the parameters on all these equipments simultaneously.
I found something about this on the following website, but may be on Honeywell website you will get more details
https://www.ainonline.com/aviation-news/2014-10-01/faa-orders-replacement-737-and-777-display-units
So the  trouble of interferences is specific to phase 3 DU, phase 3A corrects this effect.


text:
Space Catapault Model
A secretive space catapault is said to be in progress.  For the sake of mathematical simplicity, assume the launch vehicle is one kilogram point-mass and to be placed in geosynchronous orbit.
What quantitative forces (impulse?) and energy necessary to accelerate said vehicle into geosynchronous orbit?  Feel free to make any assumptions necessary to facilitate an answer.

This would probably be a better fit for space.stackexchange.com.

Maybe read about escape velocity

Notice how time lapse photos of orbital rocket shots go up to gain altitude before arcing over to gain speed.  As with aircraft, atmospheric drag is a huge issue.  Good link and very interesting projectile design.  Similar concept to "space cannon" tested around 50 years ago.  Geosynchronous orbit? That's around 22,000 miles up.

@TerranSwett  I have tried to delete the question & copy it  to space.stackexchange.com , however, the system prevents deletion.  Do moderators have the ability to move migrate the question?  If so, please to migrate

I won't be displaying mathematics or even a guesstimate here because this subject is somewhat beyond my scope of competency. However:
To shoot something into orbit with a catapult from ground level is not feasible, as the object would immediately burn to ashes, no matter what the material would be. The required velocity to reach orbit is about 40 000km/h, and to make things worse, you'd have to reach that speed at basically zero altitude, with a very dense atmosphere creating friction.
See this question and answers from physics stack exchange for further information.

See also Operation Plumbbob https://en.wikipedia.org/wiki/Operation_Plumbbob#Propulsion_of_steel_plate_cap - which may or may not have launched a 900kg steel plate into space, but definitely isn't practical for a useful payload.


text:
Could an airliner be as efficient lower and slower?
Could an airliner ever be as efficient at 15 or so meters using ground effect as it is at 50,000 km even if it has to go slower and be shaped differently?

I fail to understand if you ask for lower speeds (see Why are jet aircraft never designed with a slower cruise speed? for lower altitude (see Can ekranoplans (GEV) be more efficient than traditional airliners?)

I guess there's a typo at the higher mentioned altitude, 50,000km is kinda high for an airliner, that's beyond geosynchronous orbit

@Jpe61 -- My understanding is that the altitude at which orbital flight is possible varies with velocity.  "Geosynchronous" simply means that the  velocity required to maintain orbit exactly coincides with the velocity required to maintain a fixed position over the earth's surface.  This velocity tends to be rather high.  If we dropped an airliner moving at, say, 500 mph into a an elevation of 50,000 km over the earth's surface, it would certainly not be in any sort of orbit.  Rather, it would plummet like a rock.  So, there is no reason to assume that "50,000 km" is necessarily a typo.

Presumably the question is envisioning that the airliner at 50,000 km is experiencing negligible aerodynamic lift, but is also not moving fast enough to be in orbit.  In such a case, some (likely most) of the aircraft's weight must be supported by the thrust vector alone, so the aircraft must be flying in an extreme nose-high attitude with engines at full throttle and possibly with some sort of additional supplemental afterburner installed.  This seems rather inefficient.

Pretty obvious to me he means 50000 ft, or maybe, and a bit more mistakenly, metres.

@quietflyer Regardless of the speed of the... ummm... spaceliner(?), if it was at an altitude of 50,000 km it would be above geosynchronous orbit. It might not be in one, that's for sure. Regarding the speed you mentioned, would it be groundspeed, it certainly can't be airspeed at that altitude, so we can't reference that  At a groundspeed of 500 mph the spaceliner would be distancing itself from earth. And furthermore, since the question does mention "airliners" and "50,000km", surely there is a typo somewhere.

Yes, especially if you shape it like a train or ship, and run it about 15 meters lower :-)  But unless you have a fairly large dry lake handy, flying at 15 m AGL is not something you want to do (at least without a lot of military training & equipment).  Your passengers, people on the ground, and the owners of the things you hit will probably object.  Maybe you can do it if you're the Soviet military...

It's important to note that an airliner cruising at 50000 ft (I assume that's what you meant, and that's actually business aircraft territory; airliners are generally below the low 40s).  Is going pretty slow from an indicated airspeed perspective.  At 50000 ft an airplane is going roughly twice its indicated airspeed (it's actual speed through 3D space is twice the dynamic air pressure acting on it - if you stick your hand out, you'll feel the force of a 200 kts acting on your hand, even though you're actually going 400 kt).  So, such an airplane is actually optimized to only fly at 240 kts anyway, if it's cruising at a true airspeed of 480 kts.
What it means is, however you want to optimize for sea level flight, the higher you go the faster you can go in reality for a given fuel burn, whatever the configuration, burning far less fuel per unit of distance in the process.

It depend whether the airliner is propeller driven or jet powered.
Propellers are most efficient at low altitudes, but air resistance decreases with pressure as you go higher, enabling the same thrust to push the plane faster. Propellers are at their most economical (very roughly) around 10-20,000 ft.
Jets are most efficient at moderately high altitudes, where the air is considerably less dense so the same thrust makes the plane go faster still. But fly too high and the air gets so thin the plane begins to struggle. So jet airliners are at their most economical (equally roughly) around 30-40,000 ft.

Could an airliner ever be as efficient at 15 or so meters using ground
  effect as it is at 50,000 km even if it has to go slower and be shaped
  differently?

Yes it can.  The most efficient shape for an airliner is no shape at all, so that the passengers have to get out and walk.  (On 15-meter-tall stilts of course.)
Alternatively, consider attaching electric motors to the wheels, so that the airliner could taxi to its destination along established roadways. 15-meter-tall landing gear could be used to maintain the prescribed altitude. "Stub" wings could be retained, so that the aircraft could be claimed to be "using ground effect" to some infinitesmal degree.  This mode of transportation would surely be more efficient, in terms of energy expended per unit mass transported per mile, than climbing to 50,000 km, at least for any total distance equal to less than one half of a complete orbit of the planet.
Unless, of course, the airliner is assumed to be magically transported to 50,000 km before we begin the calculations to establish efficiency.  If this is the case, the airliner can follow a ballistic trajectory toward its destination and then, upon entering the atmosphere, transition to gliding flight.  In this particular case, flying between the same two geographic points while maintaining an altitude of 15 meters AGL would likely represent a greater expenditure of energy than following the ballistic/ gliding profile described above.

your chance for "peer pressure" -badge is nearing.

Cool!  Just remember, its not kosher to change a question in a way that invalidates an existing answer.  Anyway-- this whole answer is meant to be a gentle suggestion that the original question was insufficiently constrained, and also probably contained a typo, which had not yet been fixed at the time the answer was posted.

To glide, there must be air.

I upvoted you...

How should it read? I really like your answer and if I can reask this as a new question and leave this one as is then you can use your answer there?


text:
How does MCAS compare to stall protection systems installed in other aircraft?
How is MCAS in 737 MAX 8 different from the stick pusher in MD80 or DHC8 and from the Airbus alpha protection, in terms of overall safety?
All these systems will push the nose down at high AoA and all of them rely on the AoA data, which can become corrupted?
If all of those systems perform a similar function, why is only MCAS currently under scrutiny?
(this is an operation safety question, not related to Boeing's apparent lack of proper disclosure and training)

They activate under different scenarios. Only MCAS is under scrutiny because it's the newest of those therefore it may require some initial updates like all software once released.

Just being pedantic here, but an important distinction: MCAS is not a stall protection system.

@Radu094 It's an excellent point, not pedantic at all. The purpose of MCAS is not stall protection, and comparing it to "other stall protection systems" is like comparing Valencia oranges "to other apples." VTC.

I did not imply that MCAS is a stall-protection system; my original question was changed by en editor. Regardless of its purpose, the outcome of MCAS is a nose-down attitude change, similar to the other two systems. I was wondering why MCAS is currently considered evil, but the other two are not.

What MAY have happened (currently under investigation) is the MCAS kept pushing the nose down beyond the ability of the elevator to compensate, especially after it was turned back on.  Sadly, at this point, the aircraft may have been actually stalled, but machines can only do what they are programmed to do and it kept firing.  There was some discussion of programming logic on this site, hopefully MCAS "brain" can read other (elevator) inputs and abort/reset. It needs to be tied to other systems (including pilots) to better determine if stall is "real".

Hopefully the engineers will really go at this one and consider all options.  A small forward canard would make a great MCAS.  Making the Hstab larger with more elevator may help too.  Remounting the LEAPS Citation 550 style should not be out of the question either.

@RobertDiGiovanni Those changes would be radical enough that Boeing couldn't have gotten away with calling it a B737 anymore, which was the entire point of the MCAS compromise.

Deadlines and budgets are the biggest enemies of good engineering.  Especially since the 787 is doing well.  But they made the 737 decision years ago when things were not so clear.  They are probably big enough to absorb this, and the silver lining may be a new round of design developments, including an LNG version, which will fly for a lot less.  Us oil shortage era folks must make way for a new era of plentiful cheap fuel, which will allow some style in aviation designs once again.

The main difference is that the pilots can overcome "stick pusher" systems used in other aircraft by pulling back on the yoke/stick, though that may be difficult depending on how exactly the system works.  (Some use the standard auto-pilot servos, which are relatively easy to overpower, but others make it more difficult.)
MCAS operates by adjusting the horizontal stabilizer trim, which cannot be overcome by the pilot pulling back on the yoke, no matter how much force is applied.  In the two noted accidents, the pilots did try countering with opposite trim several times, but as soon as they stopped, MCAS kicked back in to undo their corrections.  They did not follow the emergency checklist for runaway stabilizer trim as Boeing/FAA expected, which would have disabled MCAS as a side effect.
There's also a separate problem where MCAS normally uses a single AOA sensor, which goes against aviation engineering principles of having redundant everything.  Boeing did offer an optional feature to compare multiple AOA sensors and alert the pilots if they disagreed, but neither of the two accident airlines purchased that option.  One of the proposed fixes is to make that a standard feature, both for all new production and mandatory retrofit of all existing planes without it.

Sorry, but this is inaccurate. The MD-80 has a dedicated hydraulic stick pusher that requires significantly more force than the autopilot servos. The Dash 8 uses pressure from a nitrogen tank. Both do not use the AP disconnect but a separate switch. They are intentionally difficult to override due to the risk of a deep stall. Airbus FBW uses an completely unoverridable software command.

Updated.  Thanks for the info!

How does MCAS compare to stall protection systems installed in other aircraft?

Compared to Airbus A320

Boeing didn't initially inform pilots or provide training materials for pilots explaining the existence, purpose, effects and means to control MCAS. AFAIK Airbus have always provided pilots with training materials and other information about flight envelope protections and flight control laws (normal, alternate, direct etc).
MCAS used input from only one AoA sensor of the two available. AFAIK Airbus A320's flight envelope protections use inputs from three AoA sensors and can detect when a single AoA sensor is giving erroneous readings and disregard the faulty sensor.
MCAS will repeatedly attempt to make corrections even after it has initially been temporarily disabled by a pilot's control inputs. I don't think this applies to Airbus A320s - If flight envelope protections are disabled I believe that means transitioning to a different flight control law without any possibility of automatic and unannounced reversion to normal law.

Related questions on Airbus stall-protection or flight envelope protection

Is it possible to totally override the flight computer on the Airbus A320-family (in simple non-technical terms)?
Will an Airbus A320 plane go into an unrecoverable nosedive and crash if a single angle of attack sensor fails, like on Lion Air JT610?

Other

Flight Control Laws, Skybrary

MCAS is currently under scrutiny because it has apparently caused two crashes resulting in 346 fatalities. Other similar systems have undergone scrutiny in the past. For example, the flight control system in the Airbus A330 was under the microscope after suffering a number of very similar failures. On Qantas flight 72 the computer received faulty AoA data causing a sudden nose-down. They were fortunate enough to have enough altitude to recover, but it resulted in some serious injuries. It was determined that the incident was not completely isolated.
As to why the A330 was not grounded where the 737 Max was, that is probably due to the fatalities involved. Airbus just dodged a bullet in that the crews had sufficient time to recover. Timing was everything.

The OP seems to ask on technical differences, not on the manufactures or regulators response or the impact on carriers. A look at ATSBs final report on QF72 reveals a significant different approach regarding fault tolerance.

@Andreas The question in the title is slightly different from the one in the body. I was answering “If all of those systems perform a similar function, why is only MCAS currently under scrutiny?“ That seems to be what he’s getting at according to his comments.


text:
How does the HUD Display System work?
I have seen a couple videos of fighter jets and commercial planes with HUD systems and was wondering how do the heads up displays work?

http://en.wikipedia.org/wiki/Head_up_display

HUD
The most common form of head-Up Display (HUD) is simply an out-of-sight display screen reflected in a transparent flat sheet of glass in the viewers line of sight out the cockpit front window.
The glass sheet is called a combiner. It combines the HUD information with the view out of the aircraft.
There are coatings for glass (E.g.) which allow for reflecting a monochromatic image while allowing transmission of other colors.
The display is obviously tailored for that specific use. It must avoid obscuring the view. Therefore it mostly consists of minimal light-colored lines and numerals/letters against a dark background. The dark background allows the pilot to see through the reflection to brighter objects outside - it's a bit like the matt black upper surface typically found on many car dashboards which prevents distracting unwanted images of the top of the dash being reflected into the driver's view out of the windscreen.
Here's an example from Boeing

A HUD display is visible on the left in the pilot's line of sight. Above it, mounted on the ceiling is the projector unit.
The Projector contains optics to make the pilot's view of the display appear to be at infinity (rather than the few feet away it is in reality). This allows the pilot to read the data while their eyes are focused on far-away real-world objects outside the aircraft (runways, weather, other aircraft etc).
EFVS
Beyond the projection of basic flight instrument data is the use of HUDs for Enhanced Flight Vision Systems (EFVS) which provide a kind of augmented reality display that typically uses external infra-red camera systems to identify landmarks (e.g. runway outlines) that may be obscured to the pilot due to fog etc.

Very nice improvement, thanks!

How do these systems control for parallax error? Or I assume the ones on commercial flights don't display any info that relies on precise head position?

@RoboKaren: The lens that makes the display appear at infinity happens to correct for parallax error as well. The optical element creates virtual image at infinity and in behaves as if it was really there in all regards.

You actually don't need a special coating, though it helps; the angle of the glass itself, coupled with a well-lit projection, is enough. The concept and implementation are similar to the "Pepper's Ghost" illusion.

Thanks for the simple explanation. But I have seen in some cars a glass that reflects all the colors, yet it's transparent. Can you explain why?

@zagoku. cheaper combiners use partially mirrored glass, this reflects all colours but has the disadvantage that it attenuates light from the outside world to a degree that is unacceptable for piloting aircraft. See link in answer re coatings.

authors: Дмитрий Филатов


text:
Is one-sided thrust reverser deactivation still allowable?
While browsing the major accidents in the Aviation Security Network's database, I noticed that a couple of accidents seem to at least be aggravated by one-sided deactivation of thrust reversers. One-sided thrust reverser deactivation seemed to be permissible according to the respective Minimum Equipment Lists, yet the reports indicate that the crews have been caught off guard by the resulting asymmetric thrust after reverser activation during landing.
As some of the mentioned accidents resulted in a high number of fatalities, I wonder if the procedures and requirements regarding thrust reverser deactivation have ever been changed. How common is it for an airliner on a commercial flight to have thrust reversers deactivated asymmetrically nowadays?

Operating with a deactivated thrust reverser is relatively common on normal line ops.
There are normally type specific operating restrictions e.g. full power take-off, no short or narrow runway ops.
On B757/76 there is a clue that you a have a reverser locked out - you can't select it.

any source/reference/picture?

The technical aspect of my answer is from company controlled documents so I'm unable to copy anything here. From my personal experience its not all that uncommon to encounter locked out reversers.

this information should be also on a pilot manual, for example, and usually those a publicy available, aren't they?

I suppose that depends on the type of operation. The question asked about how common it was to encounter a deactivated reverser. Type specific limitations would be direct by an operational requirement from the MEL. General operator requirements for particular situations(Narrow or Contaminated runways) are detailed in the individual operators manual.

The B737 MEL say :

Use of reverse thrust is left to the discretion of each carrier. Techniques for controlling the aircraft with unsymmetrical reverse thrust should be developed and used in training.

Therefore the pilot must know if he is able to control the aircraft in this situation.

It seems to me that the OP is aware of what the MEL says. I have the feeling he might be more interested in actual regulations (e.g., FAA or EASA)

@Frederico is right. In addition, if major carriers have issued internal guidelines on this topic, this would be of interest as well.

The MEL is approved by some authority (EASA in this case), so we can assume that this is the EASA regulation.
Edit : This is issue from EASA DDG (dispatch deviation guide), section MMEL (master minimum equipment list) for B737NG.
This the a part of the operation procedure in case of allowed dispatch with one thrust reverser inoperative.
I don't know the airliners procedures, so I can't help you in this side.


text:
Are there any rules regarding construction near a private strip, such as stand off distance?
I would like to construct a facility next to a private strip.  What are the FAA rules concerning this such as stand off or height?

the approach and takeoff chart may need to be adjusted depending on the height

@ratchetfreak do private strips have charts?

At the very least you should do any construction as a good neighbour. So no placing a 200ft tower right in the approach path...

Best is to contact your FAA regional office as the details of the procedures seem to vary a bit depending on your region, and the use of the airport.
AC70/7460 gives guidance the lighting and lighting of obstructions that may affect the airspace. The first paragraph reads:

A sponsor proposing any type of construction or alteration of a
  structure that may affect the National Airspace System (NAS) is
  required under the provisions of Title 14 Code of Federal Regulations
  (14 CFR part 77) to notify the FAA by completing the Notice of
  Proposed Construction or Alteration form (FAA Form 7460-1). The form
  should be sent to the Obstruction Evaluation service (OES). Copies of
  FAA Form 7460-1 may be obtained from OES, Airports District Office or
  FAA Website at http://oeaaa.faa.gov.

More information can be found here   here.
The reporting requirements are laid down in CFR 14 part 77 §77.9


text:
Are there any reference values for Cl and Cd of a NACA 0012 airfoil to verify CFD simulations?
I am having trouble finding the established results of inviscid flow CL and Cd variation over NACA 0012 airfoil, for the transonic region. I need the data to verify the results I am getting through my CFD simulations. I have recently started learning CFD and the actual results would be really useful to me as they would clarify whether I am in the right direction or not.
Please suggest any site, research paper or book.

Did you try scholar.google.com? The first hit when searching for 'NACA 0012 transonic' delivers some windtunnel results

The inviscid c$_D$ of a NACA 0012 is the same as for all other airfoils: 0!

Four sets of measured data can be seen in this plot, the difference caused by different Reynolds numbers during measurement. The figure is from Helicopter Performance, Stability and Control by Raymond Prouty. The book is still for sale and is a treasure cove of knowledge on all things sub-sonic. From page 426 onwards it has a section on representing airfoil data with equations, using NACA 0012 as example.

This graph is from Theory Of Wing Sections by Abbott & Von Doenhoff. Contains a chapter of compressibility effects.
$C_L$ and $C_M$ are more extensively represented in the reference books than $C_D$, which is more generally lumped into considerations of Mach divergence number. The same two books will help you further.
The NACA technical report server can be of help as well, a starting point there would be NACA TR 832.


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What are the effects of endplate on airfoil section?
A 3D wing cannot achieve the same high lift/drag ratio (more than a hundred sometimes) displayed by a 2D profile (airfoil section).
This is due to 3D effects like spanwise flow, wingtip losses(vortices) etc.
What is the lift-to-drag ratio on a wing between endplates? Can you achieve the same ratios as a 2D section?

I dimly remember reading years ago that to get close to that kind of effect from an end plate it has to extend about for about a chord length beyond the profile all the way around.  In other words, it would be 2 chords +wing thickess high, and 3 chords long.  It would make a nice billboard.

thx. under those circumstances(and assuming small enough tip clearances) the wing would behave as "extrusion" of the 2D profile, correct? in other words, no wingtip losses, minimal spanwise flow etc.

@JohnK But then you'll have an increased interference drag and skin friction drag. Maybe you will recover the 2D lift slope, however?

Yes I recall that you get close to it theoretically but the drag of this giant sheet on each wing tip negated any benefit... which is why, as usual with these types of devices that only have a theoretical benefit, end plates are just about never used, and when you do see them they are usually too small to really do any good.


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What is the pivot point of pitch change on an aircraft?
I was wondering if there is a way to determine the pivot point when an aircraft changes pitch, assuming a standard slightly nose heavy and back elevator trimmed Cessna 172.  Will it pivot around the CG, the Center of lift, or somewhere in between.
Logically, I would think it would be "from where it is held up", but some writers stress CG.
Can you help me understand?

By the very definition, a free-flying object can only pivot around its CG.
Using anything else as a pivot (e.g. Aerodynamic Center) involves a combination of momentum and translation (linear movement) of the CG and will quickly get you into mathematical problems once you start combining the forces acting on the object. Make no mistake, an object only rotates around its CG.
If this is just a mental visualisation exercise, you can 'imagine' the pivot point to be at the center of pressure (i.e. the famous plane suspended by strings at the wings), and as the plane is already flying at a few hundred knots in +x, the little +y acceleration error will not make much of a difference as a mental exercise, but the error is there mathematically.
Later edit:if you still have problems visualising it, place your aircraft in outerspace where there is no air or gravity.Put some big thrusters on the wing and smaller thrusters on the tail. CG (center of mass) is very much forward of the wing. Now fire the thrusters.

I think this is an oversimplification. Consider a car on a rail. This will not rotate around the CG. In other words, when there's a direct relation between pitch and vertical speed, the object no longer necessarily rotates around the CG. (Of course, it's much easier to consider rotations around the CG and translation of the CG separately, but the superposition of rotation and translation may give an apparent rotation around a different point)

@Sanchises a car on a rail is constrained, while a plane is not. I agree that this is a first order simplification, but your counterexample does not really help, imho.

exactelly, there is a difference in that the rail adds a constraint for the car to follow a (solid) track through the Normal force, which the air arround the wing does not. Of course, you are right, it is a simplification only, you can mathematically choose any point as the pivot, and describe all movement as a combination of rotation and translation; it is just easier overall if you pick the CG

But that only applies if it was a free floating object.  So that would be the case for an airplane doing a "bunt".. a ballistic arc.  But the airplane is "resting" on something, the center point of the lifting force resisting gravity.  You still have a center of mass trying to rotate the overall mass about the center of lift, opposed by the tail downforce.    It's the C of G that is the application point of the nose down pitching moment.  The pivot axis that force is acting on has to be aft of that, at the center of lift.

Yes indeed!  Difference in "weightless" and free flying.  Anything resisting gravity pivots around the resistance. People standing don't rotate by their CG when they fall over. A PLANE IN FLIGHT HAS WEIGHT AND IS RESISTING GRAVITY, just like a parachute.

@JohnK as I say in my answer, that depends on your frame of reference. you can define one that rotates around the CoG, or around the CoL; neither is wrong, they will simply lead to different equations. CoG equations tend to be simpler.

@robertdigiovanni People standing don't rotate by their CG when they fall over. Oh, but they do. That center of gravity is falling and moving forward from momentum. But the rotational aspect is still around their CoG.

Tom, respectfully, if they were supported against gravity by their feet, in air, they would continue falling until they were head down.  A bit like a tumble in a hang glider.  No different in an aircraft, unless it is in orbit, CG goes low, lift (or gravity resisting drag) goes high.  Let's do three cases with a good old lawn dart.  1.  Weightless, no air.  Rotates around center of mass. 2.  Weightless with air.  Rotates between CG and feathery end?  3. Air and Gravity  Goes nose down stable dive in a hurry.  Little bit different, no?

@RobertDiGiovanni there is no difference between orbiting vs flying vs walking on the ground. There is no such thing as ‘resisting gravity’ or ‘airplane resting on something’. There is an accelerated frame of reference and an unaccelerated one, and (as you have repeatedly been told) it makes no difference in which one you are as supposed to your GC/pivot point. I understand your intuition is playing tricks on you now, but you need to believe the math and make an effort to convince your imagination that a flying plane rotates arround the CG.

much like that monocromatic gif with the spinning lady, it takes a while to convince your mind the lady is spinning in the other direction. Once you do it, then you never understand why you thought it was spinning in the opposite direction. Very much the same thing here with the pivot...

Radu, with more respect, there is no such thing as resisting gravity?  If you are not accelerating at 9.8 m/sec2 on earth you are resisting gravity.  I do believe mathematical (and computational) models are very valuable, but one must not ignore the natural phenomena they describe (and predict!).  I believe they have it tougher in weather science.  Keep up the good work!

of course, you can call it resisting (though inventing new terms is not a good ideea), but the result is that you are not accelerating. there is no magical thing happening that makes the airplane rotate differently. In space or in level flight a body will act the same when a force is applied..that’s my point

Interestingly, and with your acknowledgement of the unaccelerated frame, let us move on to orientation.  Straight and level, it hangs by its wing, nose down, it will rotate by its CG. A good many airmen were lost in power dives because now the wing torques against the elevator. Weight too far forward sometimes is not good.

yes, there is a forward limit to CG as it reduces elevator effect. If you are trying to imply that the plane will pitch nose down because of a forward CG, I can also point out that a forward CG means the wing is further 'back'. It is not the nose that pitches down, it is the wing and tail that are 'lifting' upwards. See how that works?

Yes, I do.  And I also believe the CG model is very good for Low wing dihedrals such as the Piper Cherokee.

But we must consider horizontally, the wing lift is opposed by gravity, vertically it is not. Please draw in the gravity vector too.  I had proposed pulling out of the dive inverted, may save a drone.  Thanks all.

It depends on your frame of reference
You have not specified a frame of reference, so I will look at the two most common choices.

Body-fixed frame of reference

This is the frame that moves with the aircraft. By definition, any rotation happens w.r.t. the origin point of such frame. By standard practice, the origin is the center of gravity (the equations are much simpler). Thus in this case Radu094's answer applies.

Earth-fixed frame of reference

In this frame, not only you are pitching, but you are also moving forward (in the case of fixed wing aircraft; if you are in a hovering helicopter, then it is still the Center of Gravity).
This forward motion completely changes the position of the "pivot point", that will end up being outside the aircraft.
Assuming a vertical load $n$, and a forward velocity $V$, your aircraft will pull up around a point placed $R$ meters above the aircraft when the aircraft started pulling up:
$$R = \frac{V^2}{g \cdot (n -1)}$$

Consider a rectangular wooden board that you hold at one end. Place in a stream of moving water and hold at one end.  It will align downstream.  Now let go and watch it twirl merrily downstream, AROUND ITS CENTER.   Got it?

@RobertDiGiovanni you are not holding the aircraft though. your example has the same problem of Sanchises' car. also, the "around its center" is in the body fixed reference frame, because in the earth-fixed one the center is moving, thus you say that you agree with my answer.

@RobertDiGiovanni also, if you want to focus on the "person falling forward" analogy or similar, then you want to focus on the earth-fixed reference system, i.e. the second part of my answer.

Standard practice?  This is what is being studied. The important distinction here is any object resisting gravity is NOT weightless and there for is held up at a point we know as center of lift.  Your second example shows G force being created by a turn.  I accept it.  Fortunately for us, CG and CL are pretty close in aircraft, but this may make further study of interest.

Pitch is an axis in the six degrees of freedom. Therefore that defines it as being a body-fixed frame of reference. Your earth-fixed frame of reference doesn't make sense to me.

@TomMcW I can still observe you pitch up from the ground

@RobertDiGiovanni you will discover that if the CL is far from the CG, you will have problems flying. and that the equations remain simpler if written for the CG

@RobertDiGiovanni I would appreciate it if you would stop typing words in all-caps. It comes across as condescending, like you think you need to slow down in order for us to understand what you're saying.

The pivot point is whatever you choose! However, choosing the centre of gravity makes any analysis simplest.
A motion of a free moving body (e.g. an airplane) can be described as combination of motion of an arbitrary reference point and rotation around that point. As far as describing the path taken by the body, any reference point will do.
However, for dynamic analysis, that is understanding the relation between the forces acting on the body and its movement, using centre of mass (a.k.a. centre of gravity) is the only reasonable option. That is because both the Newton's Second Law of Motion,
$$ a = \frac{F}{m}, $$
and its rotational version,
$$ \alpha = \frac{\tau}{I} $$
($a$ is acceleration, $F$ is force, $m$ is mass, $\alpha$ is angular acceleration, $\tau$ is torque and $I$ is moment of inertia), only work in these simple forms for the centre of mass (you can verify that by doing the integration over the mass of the body).
Alternatively in some cases you might want to chose the pivot point so that it is not accelerating, but depending on the path taken, that may not be possible. For example when the pilot pulls on the control column, an downward force will be generated at the tail. Its moment will cause upward angular acceleration in pitch, but it is still an unbalanced force, so it will cause a downward acceleration of the centre of gravity first. This combination means a non-accelerating pivot lies somewhere far ahead of the plane.
But as the attitude and path change, the angle of attack will increase and the wings will build up an upward force that eventually balances, and exceeds, the downward force at the tail and the plane will accelerate upward. At which point, a non-accelerating pivot lies somewhere far above the plane (see the diagram in Frederico's answer). So in case of an airplane, looking for a non-accelerated pivot is futile. It rarely works in situations other than where the pivot actually has a solid support, so it can't accelerate.

Ahhh now I get what you are saying.  Now it make sense.  Thanks.

It is important to realize, accepting the center of gravity model, that the upforces in the wing and H stab generated by deflecting the tail down are instantaneous with with the tail deflection.  The plane will not sink, then rise, it pitches up and climbs immediately, putting more down pressure on the tail with vertical motion.

@RobertDiGiovanni, no, the force on the h-stab is instantaneous with the tail deflection, but the force on the wing is not, because that is a function of the angle of attack and the force on the tail needs some time to act on the plane to change its attitude and velocity. So the plane might actually sink a tiny bit first (deflecting the elevator takes a bit of time too, and alpha will start building up when the torque does, so it might not actually sink, but there is a distinctive lag between the forces).

Agree there, this is why I like to test to see how much.  Interesting to look at conservation of energy in this system.  An aircraft in flight is in dynamic equilibrium, as opposed to being static.  Does adding elevator input energy to the system, or is it altering its orientation in the wind field only?

@RobertDiGiovanni, deflecting the elevator does not itself add power, but it creates drag, so it changes the distribution of energy.


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Do modern airliner wings suffer from a "gap" in safe AOA flight?
The supercritical wing is really great for cruising at high Mach numbers with little change in AOA.

Slats do not create more lift at constant AoA. They allow for higher AoA without stalling.

The problems with MCAS have all had to do with its failure modes - i.e. it handled a failed AoA sensor very badly.  With valid inputs, MCAS activates only in rare circumstances and its presence doesn't suggest anything even vaguely like what you posit. No, there is no "gap" as suggested.

this question has 3 answers, that's why the system will not let you delete it. a moderator needs a good reason to overrule the system. such a reason seems not to be present here, that's why your request cannot be fulfilled. ask on meta if you want to push it further.

It's kind of a solution in search of a problem.  Slats create massive drag and some vibration and there is simply no point in leaving them extended at high speeds when there is plenty of Alpha margin with the clean wing. There really isn't any point.
You have an airplane that typically stalls clean at around 120-140kt.  You are cruising at 240kt indicated at the low end, maybe up to 290-300 kt indicated at the high end. What is the point of having slats out?  If the airplane has nasty stall behaviour, well that's the stick pusher's job.
In any case, if you built an airplane with extendable slats that can handle Vmo, which is what you would have to do, forget about being able to get to Vmo in the first place, unless you had afterburners or something, and then you'd have to replace half the passengers with fuel tanks.
If you really wanted to do something like that, you wouldn't build slats that stay out at high speed, you would just make the leading edges slotted, which used to be a popular thing on light aircraft in the 40s.  Still, on a high speed aircraft a massive drag producer, and possibly Mach related issues too with slightly accelerated air flowing up through the slot - who knows.
Even the light aircraft business had abandoned LE slots by the mid 50s except for special STOL cases.

I'm all for supercriticals in cruise!  They are the best high Mach subsonic wing going, but begin to lose what little lift to drag ratio they have at 5 degrees AOA, many stalling past 12 degrees.  This is clearly not a maneuvering wing, Whitcomb himself added camber to them to improve lower speed/higher AOA performance.  The suggestion is to use power and more camber to improve climbing efficiency  at lower AOA to stay away from stall.

No it is horribly inefficient and again there is no practical point.  The drag of slats is absolutely massive.  You aren't going around maneuvering in combat pulling 7G and loading the wing to near stall AOA at high speed in an airliner, and that would be the only point.

"Given recent concerns and extensive efforts to monitor and control AOA (stick shakers and MCAS), would it make sense to look into developing slats that could remain deployed up to cruising altitude"-- no it would not.

This really isn't an appropriate answer, please add some explanation.

I don't disagree, but it would be appropriate to explain why it would not.

It's it kind of logical: if that was a better design, why not make the high lift devices part of the main wing anyways? If they are "needed up to cruising altitude", they obviously aren't that bad on cruising altitude (it's not like something magically changes). So to make it cheaper just use HLD all the way and make them static.

This may not be the answer this question needs, but it certainly is the answer it deserves.

Flaps and Slats are high drag devices and only added where they are needed to reduce landing and takeoff distances. Their real purpose is to reduce approach speeds. They do this by changing an efficient high speed, low drag wing into a slow speed, high drag wing. To use them in cruise would make the aircraft LESS efficient, not more efficient.
While most aircraft have Flaps, and many have both Flaps and Slats, some aircraft have NO Flaps and NO Slats. (These are generally low speed aircraft where takeoff and landing distances are not a factor)
Any aircraft can land just fine without Flaps and Slats. This is routinely demonstrated in Simulator training for Flap and Slat failures. Without Flaps and Slats the approach speeds are much higher, and this which results in much longer landing distances. But when correctly done it is 100% safe to land without Flaps and Slats.

Slats are specifically designed (going back to the Sabre Jet(and further, if you look)) to reduce the AOA of a swept wing tip, preventing it from stalling first.  But certainly we can agree, while higher flaps ARE for drag, a lower slat and flap setting, with more power, can safely increase lift, or give the same lift at lower AOA.

The Sabre has automatic slats (basically sucked out) that only extend at high AOA, and at high speed this happens only during combat maneuvering.  Airliners don't go around pulling 7G turns at 250kt.  High speed stalls are simply not an issue from any rational safety perspective.  You are proposing a solution in search of a problem that doesn't exist.

@John K  Maneuvering Characteristics Augmentation System, MCAS, only operates in manual, flaps up flight, and returns the trim to pre-stall condition after AOA drops 0.5 below activation angle.  Hardly "a solution in search of a problem".  MCAS  "commands nose down stabilizer during steep turns with elevated load factors"  Source: Skybrary.

You are describing a special case and in any case, MCAS is NOT to correct a dangerous deficiency in the MAX's handing.  It is only to make the airplane's behaviour mimic earlier 737s.  If Boeing decided to accept a separate type rating for the MAX, MCAS would not have been required in the first place and the MAX's handling qualities would have been perfectly certifiable as is.


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Why doesn't the A380 use its outboard thrust reversers?
One thing I noticed with the A380 is that it only seems to use the inboard thrust reversers while landing. I suspect it relates to FOD ingestion or perhaps damaging the runway. This is strange though, because the 747-8 (which also has a large wingspan and very large engines), does use all of its thrust reversers during landing. Does anyone have an official answer on why this is?

I know :) but I got really embarrassed when my six yo noticed that while I have been playing FSX with him.

Why doesn't the A380 use its outboard thrust reversers?

Because it doesn't have (or need) any.

The A380 has reverse thrust on the inboard engines only. This saves weight and since the outboards are often way out over the edge of runways, decreases the risk of FOD.

From Airliners.net
Like all certified transport aircraft of this type, the A380 can stop from V1 on brakes alone with disks worn to minimum.

I've read that most airliners are allowed to take off even when all reverse-thrusters are inoperative. Most of the deceleration is handled by the brakes normally.

With the Airbus A380 weighing in fully loaded at 1,265,000 pounds, you might think stopping it within a reasonable distance after landing would require a Phalanx of Heavy-duty thrust reversers.
Truth be told, in the megaliner’s braking system, thrust reversers are the least critical components. Airliners are not required to have thrust reversers, and only the two inboard engines on the A380 are equipped with them. The decision not to install reversers on the A380’s two outboard engines saved weight and lowered the chances that those engines, which sometimes hang over runway edges, would be damaged by ingesting foreign objects.
The two reversers do help slow the A380—but not by much. In fact, unlike the thrust reversers on most airliners, including the Boeing 747 jumbo, they do not stop the aircraft in a shorter distance than brakes and spoilers alone. They do, however, take some of the strain off the brakes and are useful if water or snow makes the runway slippery.

From Airspace Magazine

More details of braking system

1.
   General
   The aircraft has:

Two Wing Landing Gears (WLG) with four wheel bogie assembly and related doors
Two Body Landing Gears (BLG) with six wheel bogie assembly and related doors
A Nose Landing Gear (NLG) with twin wheel assembly and related doors.

...

Braking
  A.    General
  Carbon brakes are installed on each wheel of the WLG and on the wheels of the front and center axles of the BLG.
  The braking system is electrically controlled and hydraulically operated.
  The braking system has four braking modes plus autobrake and anti-skid systems:

Normal braking with anti-skid capability
Alternative braking with anti-skid capability
Emergency Braking (with Ultimate Braking)
Emergency braking without anti-skid protection is also available as an alternative function of the alternate braking system.
A park brake system that is manually set is available for the BLG only. This system can also be used to supply emergency braking.

FOD risk

A380 Landing at Farnborough Airport

A380 vs 747

The design of the A380’s wings, with their large area, comparatively gentle sweep (33.5 degrees), and massive flaps, give the Airbus a landing speed that is 20 knots slower than that of a 747. An A380 crosses the landing threshold at a docile 140 knots and touches down, depending on its landing weight, at a speed as slow as 130 knots, about the same touchdown speed of some corporate jets that weigh 1/50th as much as the world’s biggest airliner.

AirspaceMag

If most deceleration is handled by the brakes, then why even have thrust reversers in the first place? I find it hard to believe that millions of man-hours (and dollars) went into designing, optimizing, and engineering entirely unnecessary equipment.

Millions of man-hours and dollars are spent on duplicate, triplicate (or more) redundant heterogeneous systems on aircraft. See Perdue University, College of Engineering

So does the difference ultimately boil down solely to the number of braked axles?

If there is fear that the outboard engines would ingest crap (FOD) because they overhang the edge of the runway, why wouldn't that also apply to takeoff, when the outboard engines are presumably at high thrust? Does an engine at reverse thrust kick air (and debris) far enough forward that there is a real risk of ingestion and FOD? Could that be negated by designing the reverse thrusters to go mostly sideways?

@Phil: That looks like a question (or three). You could click the "Ask Question" link at top right of page.

@BrysonS.: Not really. All main gear axles are usually braked. The difference is in cooling of the brakes. Every aircraft has to be able to stop without reversers, because the probability of their failure is too high to rely on them, but it might be at the cost of overheating the brakes. A380 brakes have sufficiently good cooling that they deemed two reversers sufficient.

@Jan Hudec I think my main question was why the A380 can get away with such minimal thrust reversal, while the 747-8 (and many other aircraft like it) rely on it to a far greater extent.

@Phil I second RedGrittyBrick's suggestion on making your comment a question. If you do, I'll provide an answer far longer than what I can get into a comment here.

" It can stop from V1 on brakes alone with disks worn to minimum." - All aircraft can do this, it's a certification requirement and (partly) used to determine the runway requirements. But it's really bad for the hardware. White-hot brakes can (and do) set tires on fire, and replacing half of your undercarriage  every time the pilot tries to exit on Charlie instead of Delta gets rather expensive.

@PhilPerry doesn't apply on takeoff because the jet blast is behind the engine, and it's accelerating away from the debris. Point your exhaust forward while slowing down, and you will at some point inhale any debris you kick up. Some small bizjets do use sideways "reversers" but they are more "thrust cancellers" as they do not slow the aircraft in any way.

OK, @Terry new question at [A380 v B-747 Thrust Reversers on outboard engines]

@PhilPerry Link to new question:  A380 v B-747 Thrust Reversers on outboard engines

So why does the last picture in the question show both engines on that side with deployed reversers?

@Lnafziger: Whether the pictured 747 pilots deployed all 4 thrust reversers or not on the 747 doesn't tell us much about whether or why Airbus installed 4 or 2 on the A380 - or did your question mean something else?

@RedGrittyBrick Well, I suppose that it meant that I didn't notice that it was a different airplane than what he was asking about! :)

You wrote that A380 has 10 braked axles. Note that it has 16 main wheels with brakes and 4 main wheels without brakes.

@bogl: thanks, answer updated.

@RedGrittyBrick What does FOD means?

@Gabriel: Foreign Object Damage. The outboard engines can be at the edge of the runway or over a grassed area, Reverse thrust on the outboard engines can throw debris (any loose material or objects) up in front of the engines where they will be sucked in (ingested) and may cause damage to the engine.

@RedGrittyBrick during the A380s design phase, Airbus originally wanted to do away with thrust reversers altogether - they add weight and maintenance cost to the airframe, and as all landings in commercial aircraft are required to be calculated without the use of thrust reversers, they are more of an unneeded item these days.  The FAA said no, Airbus had to include a pair.  So Airbus included a pair.  There was lots of discussion on this online a decade ago, but much of it has been lost in the annals of time.